This application claims the priority of Japanese Patent Applications No. 2001-212326 filed on Jul. 12, 2001 No. 2002-62438 filed on Mar. 7, 2002 and No. 2002-188327 filed on Jun. 27, 2002 which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a Ti-base wire rod used for forming molten metal in welding and thermal spraying.
2. Description of the Related Art
Shield arc welding is generally employed for welding of Ti-base metal members made of Ti metal or Ti alloy, in which Ti-base wire rod composed of an industrial pure Ti is used, and a portion to be weld is covered with an inert gas in order to prevent Ti from being oxidized. In a typical case of MIG (metal inert gas) arc welding shown in FIG. 5, arc AR is generated between a Ti-base wire rod 201 for use in welding and a work piece WP made of pure titanium or titanium alloy in an atmosphere of inert gas IG such as argon, helium or so. The welding is proceeded while feeding the wire rod 201 with the aid of a feed roller 202 so that the end of such rod is melted in the arc AR. A reference numeral 205 herein represents a gas nozzle (torch) for injecting the inert gas IG from the end thereof, which nozzle has at the base end thereof a flexible conduit tube 204. A reference numeral 206 herein represents an electrode chip (contact chip) fixed to the torch 205, which is responsible for holding of the wire rod 201 and for supplying electric current to such wire rod 201, WM represents a welding bead, and MP represents a molten pool. The MIG arc welding is advantageous in raising the efficiency of the welding, suppressing welding failure thanks to a deeper weld zone resulted from an improved welding energy, and facilitating welding at narrow places with a down-sized end portion of the torch 205.
On the other hand, it has been a general practice to form a coated layer with Ti-base metal by thermal spraying for the purpose of corrosion prevention for large-sized members. In the field of thermal spraying, a thermal sprayed layer is formed using a similar Ti-base wire rod as described in the above by thermal spraying process, typically by arc thermal spraying process. In the arc thermal spraying process, two Ti-base wire rods are fed to holders for current supply in parallel to thereby form an arc discharge gap between both ends of such rods, and a molten metal formed there is then sprayed using an inert gas such as nitrogen or argon, or using air as a medium so as to allow a thermal sprayed layer to deposit on a target work piece. The Ti-base wire rods are respectively fed through conduit tubes to a thermal spray gun similarly to the case of welding.
In recent years, there are accelerated trends in increasing feeding speed of the wire rod 201 aiming at higher efficiency in the Ti welding process and shorter period before completion of the welding process. In such situation, a large friction between the surface of the wire rod 201 and the conduit tube 204 undesirably interferes smooth feeding of the wire rod 201, which may at worst result in clogging or buckling of the wire rod 201 within the conduit tube 204.
In particular, surface of the conventional Ti-base welding wire rods are generally finished by mechanical or chemical polishing so as to produce a metallic gloss merely for the purpose of improving the appearance. Thus finished wire rods are however poor in feeding smoothness due to their coarse surfaces. Such wire rods having the metallic glossy appearance are also disadvantageous in that being causative of less stable arc during the welding as expected from their appearance, and desirable bead shape cannot be obtained in particular by MIG arc welding using an automatic welder since the arc trembles searching for a stable point. This may be ascribable also to that distance between the end of the wire rod and a work piece to be welded can finely fluctuate due to irregularity in the rod feeding speed.
In MIG arc welding of Fe-base members, surface of a Fe-base wire rod for welding is often plated with Cu or coated with a lubricating oil. In contrast, in the welding of Ti which is a labile metal, provision of such Cu plating or coating of such lubricating oil on the surface of the wire rod will not be practical since it may degrade strength of the weld joint due to possible formation of a brittle Cuxe2x80x94Ti-base intermetallic compound or carbide. While there is another long-established measures for stabilizing the arc during welding by introducing carbon dioxide gas or oxygen in the shield gas, such measures is still unsatisfactory since a large amount of oxygen uptake from the shield gas into the welding bead will occur, which may degrade elongation of the weld joint in particular for the case of Ti welding.
Also Ti thermal spraying essentially suffers from the same problem as in the welding. In arc thermal spraying for example, the arc is formed between two Ti-base wire rods, so that irregularity in the feeding speed of either one of such wire rods will vary gap distance for the arc discharge to thereby destabilize the arc. It can thus be concluded that the problem in the arc stability can occur more frequently than in welding.
It is therefore an object of the present invention to provide a Ti-base wire rod for forming molten metal, which is excellent both in feeding smoothness and arc stabilizing property in welding or thermal spraying, and is capable of ensuring desirable mechanical properties of the resultant weld portion, and quality of obtained thermal sprayed layer.
To solve the foregoing problems, a Ti-base wire rod for forming molten metal of the present invention is such that being serially melted from an end user heating to thereby produce molten metal comprising Ti or Ti alloy, wherein at least a portion including the surface of the wire rod comprises Ti metal or Ti alloy mainly composed of Ti, and the surficial portion of the wire rod including the surface thereof has formed therein an oxygen enriched layer having an oxygen concentration higher than that in the inner portion, and having a thickness larger than that of native oxide film possibly formed on the Ti metal in the air under ordinary temperature. The ordinary temperature is, for example, 20xc2x0 C.
Since Ti is a labile metal, the surface thereof is readily passivated in the air at ordinary temperatures to thereby produce native oxide film. The native oxide film can suppress internal corrosion of the metal, which ensures Ti metal or Ti alloy mainly comprising Ti an excellent corrosion resistance. The native oxide film mainly comprises TiO2 having an average oxygen concentration of approx. 40 wt %, which concentration is of course higher than that of the inner portion comprising Ti metal. The native oxide film thus can be rated as a kind of oxygen enriched layer. The thickness of the native oxide film is however considerably small, which is as thin as approx. 40 to 100 nm.
What is formed on the surface of the wire rod in the present invention is an oxygen enriched layer having a thickness larger than that of the native oxide film. By intentionally forming on the surface of the wire rod the oxygen enriched layer with the thickness larger than that of the native oxide film, feeding smoothness of the wire rod through a conduit tube or so will considerably be improved, which results in improved stability of arc typically during arc welding or arc thermal spraying. More specifically, this will ensure the following effects (the same will apply to a second aspect of the present invention described later):
(1) improved feeding smoothness of the wire rod through the conduit tube will remarkably reduce apprehensions for clogging or buckling of the wire rod, which successfully reduces frequency of process interruption and raises process efficiency in welding or thermal spraying. More specifically, while the coefficient of dynamic friction of conventional Ti wire rod having a polished surface is 0.5 to 0.6 or around, the present invention can desirably reduce the coefficient as low as typically to 0.13 to 0.17 or around; and
(2) improved arc stability will result in improved mechanical strength of the obtained weld portion or quality of the thermal sprayed layer. Although thermal spraying, in which two Ti-base wire rods are concurrently fed, is more likely to be affected by feeding smoothness of such wire rods and tends to suffer from disturbance of the arc, using of the Ti-base wire rods of the present invention can ensure sustainment of stable arc thermal spraying.
In view of stabilizing the arc, the above described effect (2) will be predominant especially in arc welding in which the arc is covered with an inert gas containing no oxygen. On the other hand, arc thermal spraying can be proceeded in either way such that using an oxygen-containing spraying medium such as a compressed air, or such that using an inert gas medium such as nitrogen, argon or the like, where the arc stabilizing effect (2) will be predominant especially for the case using the inert gas medium.
A Ti-base wire rod for forming molten metal according to the second aspect of the present invention is such that being serially melted from an end under heating to thereby produce molten metal comprising Ti or Ti alloy, wherein at least a portion including the surface of the wire rod comprises Ti metal or Ti alloy mainly composed of Ti, and the surficial portion of the wire rod including the surface thereof has formed therein an oxygen enriched layer having an oxygen concentration higher than that of the inner portion,
such oxygen enriched layer being adjusted so that ratio Tw/Dw falls within a range from 0.3xc3x9710xe2x88x923 to 1xc3x9710xe2x88x921, where Tw represents the thickness of the oxygen enriched layer and Dw represents the diameter of the wire rod; and
such oxygen enriched layer having an average oxygen concentration of 1 wt % or above.
In the Ti-base wire rod for forming molten metal according to the second aspect of the present invention, what is formed on the surface of the wire rod is an oxygen enriched layer having a thickness larger than that of the native oxide film, being more specifically adjusted so that ratio Tw/Dw falls within a range from 0.3xc3x9710xe2x88x923 to 1xc3x9710xe2x88x921 on the basis of Dw, which is equivalent to that thickness Tw equals to 0.03 to 10% of diameter Dw of the wire rod, and having an average oxygen concentration of 1 wt % or above. By forming the oxygen enriched layer with such thickness and average oxygen concentration, feeding smoothness of the wire rod through a conduit tube or so will considerably be improved similarly to the foregoing first aspect, which results in improved stability of arc typically during arc welding or arc thermal spraying.
The ratio Tw/Dw of the thickness Tw of the oxygen enriched layer and the diameter Dw of the wire rod less than 1xc3x9710xe2x88x923 (0.1% of the rod diameter Dw) or the average oxygen concentration of the oxygen enriched layer less than 1 wt % will result in insufficient improving effect of the feeding smoothness. Another problem resides in that the arc will be more likely to destabilize, which is disadvantageous in forming uniform welding bead or thermal sprayed layer. On the contrary, Tw/Dw exceeding 1xc3x9710xe2x88x921 (10% of the rod diameter Dw) will require a considerably long time for forming the oxygen enriched layer, and even the difficulty in the formation results in only a limited effect (which may be even harmful when the wire rod is intended for use in welding or so, since strength of the weld joint in weld structure may be degraded).
The average oxygen concentration of the oxygen enriched layer reaches maximum when the entire portion of the oxygen enriched layer is composed of titanium oxide, and the value thereof is considered as being equal to an oxygen content ratio estimated from a formula weight of the resultant oxide. For example, when the resultant oxide is TiO2, the upper limit of the average oxygen concentration estimated from the stoichiometric oxygen content will be 40.06 wt % (calculated assuming atomic weight of titanium as 47.88 and oxygen as 16.0). On the other hand, it is also allowable to form a titanium oxide having a stoichiometric oxygen ratio larger than that of TiO2, where forming Ti2O5 for example will result in the upper limit of the average oxygen concentration of 45.52%. It is thus not realistic in general that the average oxygen concentration of the oxygen enriched layer exceeds 45.52%.
When considering the foregoing effect of (2), that is, oxygen localized in the oxygen enriched layer in the surficial portion vaporizes out into the shield gas atmosphere in the early stage of the arc melting, which creates an atmosphere equivalent to that obtained when an oxygen-containing shield gas is used, to thereby stabilize the arc, it is concluded that the oxygen enriched layer is more advantageous when it has an average oxygen concentration lower than that of the native oxide film (titanium oxide: particularly TiO2). The fact that the average oxygen concentration of the oxygen enriched layer is lower than that of the native oxide film means that the oxygen enriched layer contains an area where the oxygen concentration is suppressed at a level lower than in the native oxide film, which more specifically means that the oxygen enriched layer is not fully converted into TiO2 but partially contains an area having mixed therein metal-state Ti. In an exemplary case in which the oxygen enriched layer is formed by thermal oxidation process (described later), the outermost portion thereof will have an oxygen content almost equivalent to that of titanium oxide, but the inner portion thereof will be an area having an oxygen concentration lower than that of titanium oxide and instead will have an oxygen diffused layer which comprises a Ti-base metal phase containing oxygen diffused therein (for example oxygen-stabilized xcex1 case). Oxygen in the oxygen diffused layer exist in a form of fine titanium oxide grains diffused in the metal phase, or as being dissolved in the metal phase. Such oxygen (dissolved oxygen in particular) is supposed to have a bond strength with Ti weaker than that of oxygen contained in the native oxide film, so that the oxygen is rather likely to vaporize in the early stage of arc melting, and is thus supposed to more effectively contribute to arc stabilization. In this point of view, the average oxygen concentration of the oxygen enriched layer is preferably set within a range from 1 wt % and 40 wt %, both ends inclusive. It is also preferably for the oxygen enriched layer that the thickness of the area having an oxygen content lesser than that of titanium oxide (TiO2 in particular) is larger than that of the outermost titanium oxide, and more preferably twice or more larger than the thickness of the outermost titanium oxide layer.
In order to obtain more eminent arc stabilization effect, it is more preferable that the ratio Tw/Dw of the thickness Tw of the oxygen enriched layer and the diameter Dw of the wire rod resides within a range from 1xc3x9710xe2x88x923 to 1xc3x9710xe2x88x921. In particular for the case in which the foregoing oxygen diffused layer is formed together with the outermost titanium oxide layer (equivalent to or thicker than the native oxide film (approx. 40 to 100 nm)) typically by the thermal oxidation process, Tw/Dw will be more likely to fall within the above preferable range since the thickness of the oxygen enriched layer will have additional thickness contributed by the oxygen diffused layer.
The thickness and average oxygen concentration of the oxygen enriched layer can be determined as described below. A section of the wire rod is mirror-polished, and is then subjected to in-plane analysis of oxygen concentration by EPMA (electron probe micro-analysis), in which the peripheral portion of the wire rod having an oxygen concentration of 1.2 Cm or above, where Cm represents the oxygen concentration of the core portion, is defined as the oxygen enriched layer, and an integral average value of oxygen concentration of such portion is defined as the average oxygen concentration. When the oxygen concentration has some variation along the circumferential direction of the wire rod section, concentration measurement circles are virtually defined at various positions on the radial direction of the section, and average values of the oxygen concentration are measured along the individual concentration measurement circles to thereby estimate the oxygen concentration distribution averaged along the circumferential direction. The oxygen enriched layer is finally defined as a circumferential area having the oxygen concentration of 1.2 Cm in the foregoing oxygen concentration distribution along the radial direction of the section.
The Ti-base wire rod of the present invention is available as a Ti welding wire rod for forming weld metal as the molten metal, and also available as a Ti thermal spraying wire rod for forming thermal sprayed metal layer as the molten metal.
Preferable upper limits of the values for Tw/Dw and average oxygen concentration of the oxygen enriched layer differ between the wire rod for welding and those for thermal spraying. Requirements for the strength are not so stringent in many cases for the thermal sprayed layer than those for weld joint (of course there are some exceptions), so that air can be used as a spraying medium for molten metal. In such case, oxygen concentration of the layer will consequently become high because the molten Ti metal deposits to thereby form the thermal sprayed layer while reacting with oxygen in the air, which is sufficient for practical use unless otherwise an especially high strength is required. So that as for wire rod to be employed in thermal spraying, raising the values for Tw/Dw and average oxygen concentration of the oxygen enriched layer as high as to the upper limits will never result in nonconformity when considering that the employed wire rod is oxidized under molten state as a consequence.
On the contrary as for wire rod for use in welding, an excessively large thickness of the oxygen enriched layer or excessively high average oxygen concentration thereof may sometimes result in nonconformity such as degraded strength of weld joint in thus obtained weld structure. In such case, it is preferable to adjust Tw/Dw to 50xc3x9710xe2x88x923 (5% of the diameter Dw of wire rod) or below, and the average oxygen concentration of the oxygen enriched layer to 30 wt % or below. Also in thermal spraying, using an inert gas for spraying medium such as argon or so, it may sometimes be preferable to limit Tw/Dw and average oxygen concentration within similar ranges when there is a need for forming a thermal sprayed layer with a high strength while suppressing oxidation as possible.
The Ti-base wire rod for forming molten metal according to the present invention mainly comprises Ti. It is to be noted that the expression xe2x80x9cmainly comprises Tixe2x80x9d in the context of this specification means that Ti is a component contained in a largest content, and preferably means that Ti is contained in an amount of 50 wt % or above. For the case a Ti alloy is employed, it is allowable to use various additional elements as side-components for the purpose of improving strength or ductility of the resultant weld portion or thermal sprayed layer. Possible candidates for the additional elements and preferable ranges of the amount of use thereof will be explained in the next paragraphs.
(1) Al: 9 wt % or below
Al can stabilize xcex1 phase, which is a low temperature phase of Ti, and strengthen the xcex1 phase by forming solid solution therewith. The content thereof exceeding 9 wt % will however adversely affect toughness and ductility since an intermediate phase (intermetallic compound) such as Ti3Al is formed in a large amount. The addition in an amount of 1 wt % or more is preferable to achieve a more distinct effect, and within a range from 2 to 8 wt % is more preferable.
(2) At least either of N and O: 0.5 wt % or below in total
Also N and O can stabilize and strengthen the xcex1 phase similarly to Al, where O shows more distinct effect of addition. The total content exceeding 0.5 wt % will however result in degraded toughness and ductility. The addition in an amount of at least 0.03 wt % in total is preferable to obtain a distinct effect, and more preferably within a range from 0.08 to 0.2 wt %. It is to be noted that the oxygen content herein means an oxygen content of the inner portion other than the oxygen enriched layer.
(3) Any one or two or more of V, Mo, Nb and Ta: 45 wt % or below in total
All of these are stabilizing element for the xcex2 phase, and advantageous in improving hot workability and strengthening through annealing. All of these elements are however high in specific gravity and melting point, so that excessive addition thereof will adversely affect the light weight nature and large specific strength, which are advantages specific to Ti alloys, and will make it difficult to use a Ti alloy ingot in manufacturing process due to elevated melting point of the alloy. The upper limit of the amount of addition is thus defined as 45 wt % in total. On the other hand, the addition in an amount of at least 1 wt % in total is preferable to obtain a distinct effect. Mo and Ta may sometimes be added only in a small amount in order to improve corrosion resistance of the alloy.
(4) Any one or two or more of Cr, Fe, Ni, Mn and Cu: 15 wt % or below in total
Also these elements have effect of stabilizing the xcex2 phase, and are effective in improving hot workability and strengthening through annealing. It is to be noted, however, that all of the elements are likely to form intermediate phase with Ti (e.g., TiCr2, TiFe, Ti2Ni, TiMn, Ti2Cu), and excessive addition thereof will tend to degrade the ductility and toughness, so that the upper limit of the amount of addition is defined as 15 wt % in total. The addition in an amount of at least 0.5 wt % in total is preferable to achieve a more distinct effect. Ni may sometimes be added only in a small amount in order to improve corrosion resistance of the alloy.
(5) At least either of Sn and Zr: 20 wt % or below in total
These are known as neutral additive elements capable of strengthening both of xcex1 phase and xcex2 phase. It is to be noted that excessive addition will only saturate the effect, so that the upper limit of the amount of addition is defined as 20 wt %. The addition in an amount of at least 0.5 wt % in total is preferable to achieve a more distinct effect.
(6) Si: 0.7 wt % or below
Si can enhance creep resistance (creep rupture strength) of the alloy, and improve the heat resistance. An excessive addition thereof will however degrade the creep rupture strength and ductility due to formation of intermetallic compound such as Ti5Si3, so that the upper limit of the amount of addition is defined as 0.7 wt %. The addition in an amount of at least 0.03 wt % is preferable to achieve a more distinct effect, and more preferably within a range from 0.05 to 0.5 wt %.
(7) At least either of Pd and Ru: 0.5 wt % or below in total
These elements exhibit an effect of improving corrosion resistance of the alloy. Since they are noble metals and thus expensive, the upper limit of the amount of addition is defined as 0.5 wt % while considering saturation of the effect or so. The addition in an amount of at least 0.02 wt % in total is preferable to achieve a more distinct effect.
Specific examples of the alloy composition will be enumerated below, where the composition herein is expressed as being headed by Ti, which is followed by side-components as being connected with hyphens together with numerals for representing compositions while omitting the unit xe2x80x9cwt %xe2x80x9d (for example, Ti-6 wt % Al-4 wt % V alloys is simply expressed as Ti Ti-6Al-4V).
(1) xcex1-Type alloys
Ti-5Al-2.5Sn, Ti-5.5Al-3.5Sn-3Zr-1Nb-0.3Mo-0.3Si, Ti-2.5Cu
(2) Near-xcex1+xcex2-type alloys
Ti-6Al-2Sn-4Zr-2Mo-0.1Si, Ti-8Al-1Mo-1V, Ti-2.25Al-2Sn-4Zr-2Mo, Ti-6Al-2Sn-2Zr-2Mo-0.25Si, Ti-6Al-2Nb-1Ta-0.8Mo, Ti-6Al-2Sn-1.5Zr-1Mo-0.35Bi-0.1Si, Ti-6Al-5Zr-0.5Mo-0.2Si, Ti-5Al-6Sn-2Zr-1Mo-0.25Si
(3) xcex1+xcex2-type alloys
Ti-8Mn, Ti-3Al-2.5V, Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-7Al-4Mo, Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si, Ti-10V-2Fe-3Al, Ti-4Al-2Sn-4Mo-0.2Si, Ti-4Al-4Sn-4Mo-0.2Si, Ti-2.25Al-11Sn-4Mo-0.2Si, Ti-5Al-2Zr-4Mo-4Cr, Ti-4.5Al-5Mo-1.5Cr, Ti-6Al-5Zr-4Mo-1Cu-0.2Si, Ti-5Al-2Cr-1Fe
(4) xcex2-Type alloys
Ti-13V-11Cr-3Al, Ti-8Mo-8V-2Fe-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-11.5Mo-6Zr-4.5Sn, Ti-11V-11Zr-2Al-2Sn, Ti-15Mo-5Zr, Ti-15Mo-5Zr-3Al, Ti-15V-3Cr-3Al-3Sn, Ti-22V-4Al, Ti-15V-6Cr-4Al
(5) Near-xcex2-type alloy
Ti-10V-2Fe-3Al
(6) Corrosion-resistant alloys (while available for welding, they are particularly useful for the purpose of forming corrosion-resistant layer by thermal spraying)
Ti-0.15Pd, Ti-0.3Mo-0.8Ni, Ti-5Ta
In the present invention, the oxygen enriched layer of the Ti-base wire rod for forming molten metal can be formed by annealing the Ti-base metal wire rod in an oxygen-containing atmosphere. Examples of available oxygen-containing atmospheric include not only oxygen-containing nitrogen atmosphere (including air atmosphere) and oxygen-containing inert gas atmosphere, but also gaseous atmosphere containing oxygen compound such as steam. In view of efficient formation of the oxygen enriched layer having a necessary and sufficient thickness, it is preferable to use an oxygen-containing atmosphere having a partial pressure of oxygen of 5xc3x97103 to 15xc3x97103 Pa, and annealing temperature is preferably set at 500 to 800xc2x0 C. for example. The oxygen enriched layer can be formed by, besides the foregoing thermal oxidation, embedding Ti oxide grains in the surficial portion of the wire rod, or by depositing a titanium oxide layer by vapor-phase film forming process such as vapor deposition and sputtering. The titanium oxide layer can still also be formed by well-known sol-gel process. Also for the cases that the titanium oxide layer is formed according to any of these methods, it is more preferable to form an additional oxygen enriched layer by thermal oxidation.